What I found by running runtest by hand is it fails if the file site.ext exists.
parallels@parallels-Parallels-Virtual-Platform:~/Desktop/Parallels Shared Folders/FlDist/trunk/testsuite$ runtest --version
ERROR: tcl error sourcing /media/psf/FlDist/trunk/testsuite/site.exp.
wrong # args: should be "set varName ?newValue?"
while executing
"set objdir /home/parallels/Desktop/Parallels Shared Folders/FlDist/trunk/testsuite"
(file "/media/psf/FlDist/trunk/testsuite/site.exp" line 5)
invoked from within
"source /media/psf/FlDist/trunk/testsuite/site.exp"
("uplevel" body line 1)
invoked from within
"uplevel #0 source /media/psf/FlDist/trunk/testsuite/site.exp"
invoked from within
"catch "uplevel #0 source $file""
parallels@parallels-Parallels-Virtual-Platform:~/Desktop/Parallels Shared Folders/FlDist/trunk/testsuite$ rm site.exp
parallels@parallels-Parallels-Virtual-Platform:~/Desktop/Parallels Shared Folders/FlDist/trunk/testsuite$ runtest --version
DejaGnu version 1.6.2
Expect version 5.45.4
Tcl version 8.6
parallels@parallels-Parallels-Virtual-Platform:~/Desktop/Parallels Shared Folders/FlDist/trunk/testsuite$ uname -a
Linux parallels-Parallels-Virtual-Platform 5.4.0-42-generic #46-Ubuntu SMP Fri Jul 10 00:24:02 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
parallels@parallels-Parallels-Virtual-Platform:~/Desktop/Parallels Shared Folders/FlDist/trunk/testsuite$